1. Field of the Invention
The present invention relates to a nonlinear distortion compensation apparatus.
2. Description of the Related Art
A power amplifier for amplifying a linear modulation signal is requested to have high linearity to reduce the deterioration of the transmission characteristic due to a spectrum characteristic and the distortion of a signal. On the other hand, the amplifier is also requested to have high power efficiency in most uses. Normally, the linearity and efficiency are mutually contradictory, and various distortion compensation systems have been developed to make them mutually compatible.
A predistorter system is known as one of the distortion compensation systems. The predistorter system obtains a desired signal without distortion in the output of the amplifier by adding in advance the inverse characteristic of the distortion of the amplifier upon receipt of an input signal of the amplifier.
FIG. 1 shows the distortion compensation apparatus in the conventional predistorter system.
An input complex base band signal x(t) is linearly modulated by a quadrature modulator QMOD after the distortion compensation coefficient h (|xc3x97(t)2|) corresponding to its input amplitude or power output from the mean minimum square error (MMSE) device is processed by complex multiplication by a multiplier 1. The modulated signal is amplified by an amplifier AMP, and nonlinear distortion is added to it. A part of an amplifier output signal is demodulated by a quadrature demodulator QDEM to obtain a feedback signal y(t) of a complex base band. The obtained y(t) is compared with an input signal x(t), and the distortion compensation coefficient h is updated such that an error signal e(t)=y(t)xe2x88x92x(t) can be minimized. An update algorithm can be a method of using an MMSE algorithm for minimizing a mean square error of e(t), etc. (Japanese Patent Laid-Open No.9-69733).
In the conventional predistorter system of adaptively obtaining a distortion compensation coefficient, a complex base band signal is required as feedback information for use in generating a distortion compensation table. The precision of the feedback signal greatly affects the spectrum characteristic of the output of the amplifier. In the adaptive predistorter system, a compensation coefficient is controlled such that an error between a reference input complex base band signal and a feedback signal can be minimized. In this case, since the distortion and noise added in the feedback path (a reception mixer, an A/D converter, etc., that is, although not shown in FIG. 1, an A/D converter, a D/A converter, etc. are provided before and after the QMOD and QDEM) cannot be distinguished from the distortion of the amplifier, they cannot be removed in principle. That is, the distortion and noise added in the feedback path are reflected in the distortion compensation coefficient as an error signal, and are superposed on the input signal of the amplifier, thereby reducing the spectrum characteristic of the output of the amplifier.
Especially, when the predistorter system is applied to a power amplifier for amplifying a broadband and highly dynamic range signal such as a W-CDMA, a multi-carrier multiplex signal, etc., it is necessary to include multiples of bands of the input signal for a feedback signal to cover a band of higher order distortion. Therefore, an A/D converter for including the feedback signal is requested to have a characteristic of a high-speed sample and high bit precision. However, a sufficient A/D conversion characteristic cannot be obtained with the current device performance. Thus, the performance of the A/D converter is the bottleneck of restricting the performance of the predistorter.
The present invention aims at providing a distortion compensation apparatus capable of realizing the control of a predistorter of a high-speed and appropriate amplifier independent of the characteristic of a feedback path.
The distortion compensation apparatus according to the present invention which compensates for the distortion characteristic of an amplifier includes: an adjacent channel leakage power extraction unit for extracting at least adjacent channel leakage power of a main channel signal to be processed in a distortion compensating process from an output signal of the amplifier; a distortion compensation coefficient computation unit for converting the amplitude value and the phase value of a distortion compensation coefficient into respective gene types, and obtaining the distortion compensation coefficient based on the genetic algorithm using the adjacent channel leakage power value or the adjacent channel leakage power ratio obtained from the adjacent channel leakage power value as an evaluation function; and a distortion compensation coefficient application unit for applying a distortion compensation coefficient computed by the distortion compensation coefficient computation unit as an input signal of the amplifier.
According to the present invention, since the adaptive leakage power value or the adjacent channel leakage power ratio can be used in directly computing a distortion compensation coefficient by using the genetic algorithm, the influence of the process error from the A/D converter and demodulator included in the feedback path in the predistorter system can be minimized, thereby computing the distortion compensation coefficient appropriately reflecting the characteristic of the power amplifier.